[mlr] introduce Mlr namespace and rename types (#13053)

This commit introduces the `Mlr` namespace to encapsulate all
Multicast Listener Registration related types and logic, improving
overall code organization and readability.

The following primary renames were performed:
- `MlrManager` to `Mlr::Manager`
- `MlrState` to `Mlr::State`
- `MlrStatus` to `Mlr::Status`
- Constants like `kMlrSuccess` to `Mlr::kStatusSuccess`

Additionally, methods within the newly scoped `Mlr::Manager` class
have been simplified by removing redundant `Mlr` prefixes (e.g.,
`SendMlr()` is now `Send()`, `FinishMlr()` is now `Finish()`).

External modules and tests have been updated to reference the new
scoped names.

tests/nexus/test_1_2_MATN_TC_26.cpp

@@ -150,7 +150,7 @@ void TestMatnTc26(void)
      * - Pass Criteria:
      * - N/A
      */
-    br1.Get<BackboneRouter::Manager>().ConfigNextMulticastListenerRegistrationResponse(kMlrNoResources);
+    br1.Get<BackboneRouter::Manager>().ConfigNextMulticastListenerRegistrationResponse(Mlr::kStatusNoResources);
 
     Log("Step 2: The DUT must be configured to request registration of Multicast address MA1.");
 
@@ -214,7 +214,7 @@ void TestMatnTc26(void)
      * - Pass Criteria:
      * - N/A
      */
-    br1.Get<BackboneRouter::Manager>().ConfigNextMulticastListenerRegistrationResponse(kMlrGeneralFailure);
+    br1.Get<BackboneRouter::Manager>().ConfigNextMulticastListenerRegistrationResponse(Mlr::kStatusGeneralFailure);
 
     Log("Step 4 (cont.): Harness instructs the device to updates the network data (BBR Dataset).");
 

tests/nexus/test_1_2_MATN_TC_3.cpp

@@ -194,7 +194,7 @@ void TestMatnTc3(void)
      * - Pass Criteria:
      *   - N/A
      */
-    SuccessOrQuit(router.Get<MlrManager>().RegisterMulticastListeners(&ma1, 1, &kTimeoutZero, nullptr, nullptr));
+    SuccessOrQuit(router.Get<Mlr::Manager>().RegisterMulticastListeners(&ma1, 1, &kTimeoutZero, nullptr, nullptr));
     nexus.AdvanceTime(kMlrRegistrationTime);
 
     Log("---------------------------------------------------------------------------------------");